The applicant's European Pat. No. 4233 discloses a device for the thermal regulation of an enclosure, comprising a temperature measuring circuit on the one hand, and on the other hand a circuit for the heating of the enclosure supplied from a voltage source and controlled by means of the temperature measuring circuit, the latter comprising a resistance bridge of which one branch comprises a plurality of elements having a resistance variable as a function of the temperature, and an amplifier circuit providing a control signal as a function of the difference of potential existing between the extremities of one of the diagonals of the bridge. The heating circuit comprises a plurality of voltage stabiliser semiconductor circuits, each of these series-connected semiconductor circuits being submitted to a substantially identical fraction of the supply voltage and being traversed by a heating current adjustable by means of a control transistor submitted to the said control signal. The heating action may be performed by means of transistors or of voltage regulators.
This device enables a comparatively uniform heating to be secured, at a given temperature, of an enclosure of optional size, without the need to adjust the semiconductor circuits utilized. Each transistor of the series chain is exposed to a fraction of the total voltage and to the whole of the heating current and all the transistors dissipate an identical power.
An arrangement of this kind makes it possible to obtain homogenous heating of an enclosure to the extent, and only to the extent that the same appears in a homogenous form from the thermal point of view. As will be established in the following, the applicants have observed that the enclosures obtainable in practice, in particular for the piezo-electric oscillators, display certain disparities from this point of view.
In the case in which the heating energies are spread uniformly around the enclosure, the disparities of thermal conductivity of the components situated in different areas of the same will cause unevennesses in temperature manifested to the user by an unexplainable thermal drift of the oscillator, while the temperature control error signal is practically zero.
The drift is a maximum when the external temperature is very different from the reference temperature at which the thermostatically controlled enclosure is maintained and which corresponds approximately to the maximum temperature of the temperature range specified for the operation of the oscillator, and the applicants were led to interpret this result as being attributable to the fact that the temperature of the electronic system of the quartz oscillator differs substantially from the reference temperature at which it is wished to maintain the same, which difference is actually manifested by a drift of the quartz frequency as compared to the required frequency f.sub.O, which evolves as has been observed.